As LLFA we are now looking to provide comments on minor applications, applying slightly less stringent criteria than we do for major applications. As we have been consulted, we are assuming that there is a potential flood risk on site, therefore we are considering the impact of increased run-off rates. The cumulative impacts of minor developments can increase flood risk in an area. Current processes for assessing major applications cannot be applied in the same way to minor applications as reduced orifice sizing to meet the greenfield 1 in 1 rate can increase the risk of blockages and therefore flood risk.
Minor sites should minimise the areas of hardstanding, and where hard surfaces are necessary unlined permeable paving should be used. This allows any infiltration potential on the site to be utilised, reducing off-site run-off. This is in line with the drainage hierarchy if water re-use is not possible on the site. If the site is directing water to a single point of infiltration, then we would need to see infiltration testing for that location. However, if the water is being distributed evenly across the site as the rain lands on the ground then we do not ask for infiltration testing as it is mimicking natural processes.
Information regarding existing surface water risk at the site should be submitted in order for us to make a complete assessment of potential flood risk on the site. Discharge rates should be limited to the 1 in 1-year greenfield rate or 1l/s, whichever is greater. It is also possible to restrict to flow matching rates, however long term storage will be required. The required storage volume and run off for the site can be calculated using the UK SuDS website. A 10% allowance for urban creep should also be included when calculating the required storage. Where it is not possible to meet the 1 in 1-year greenfield rate, rainwater re-use can be used to increase storage and reduce the run-off rate from the site. It should be demonstrated why this is not feasible if it is not proposed. Please refer back to the section on rainwater re-use for more information.
All water should have sufficient treatment before leaving the site in line with the Simple Index Approach found within Chapter 26 of The CIRIA SuDS Manual C753. The use of above ground features is preferable in order to provide treatment on site, and to provide biodiversity and amenity benefits. A maintenance plan detailing the maintenance arrangements of the SUDS features should be submitted including who is responsible for different elements of the surface water drainage system and the maintenance activities/frequencies.
A drainage plan should be submitted which includes the site layout, location of features, outfall location and conveyance. The drainage plan should also consider exceedance flows to ensure potential off-site flooding is managed. Engineering drawings of the components being used within the scheme should also be included.
Page updated: 17/02/2020